It is well known that present D/I communication networks comprise two end stations connected by two lines to a chain of intermediate D/I multiplexer stations. They provide communication for multiple data and/or voice channels between any two or more stations. The channel access can be in either direction, i.e. it can be directed East or West.
The problem with such networks is that when there is a break or failure within the network it must be found and repaired before the network can resume its normal operation. This may be time-consuming and while the search for the failure and subsequent repairs are carried out, the network is out of service, which is a major problem.
Some attempts have been made to provide protection to such communication networks by using protection switching, i.e. routing the traffic from a faulty channel onto a protection channel. One such system is described in Canadian Patent 1,255,770 issued Jun. 13, 1989 in which there is provided an individually controlled selector for each normal channel for selectively supplying the traffic of the channel to the protection channel. Such a system requires duplication of all channels, since for every normal channel there must be a protection channel and in case of fault the faulty normal channel is switched to redirect traffic to the protection channel. This is tedious and costly.